Methods and apparatus for determining an etch endpoint in a plasma processing system

ABSTRACT

Methods and apparatus for ascertaining the end of an etch process while etching through a target layer on a substrate in a plasma processing system which employs an electrostatic chuck. The end of the etch process is ascertained by monitoring the electric potential of the substrate to detect a pattern indicative of the end of the etch process. By the way of example, changes to this potential may be observed by monitoring the current flowing to the pole of the electrostatic chuck. Upon ascertaining the pattern indicative of the end of the etch process, for example by monitoring the current signal, a control signal is produced to terminate the etch. If a bias compensation power supply is provided to keep the currents flowing to the poles of the electrostatic chuck substantially equal but opposite in sign throughout the etch, the compensation voltage output by the bias compensation power supply may be monitored for the aforementioned pattern indicative of the end of the etch process in order to terminate the etch.

BACKGROUND OF THE INVENTION

The present invention relates to the manufacture of semiconductordevices. More particularly, the present invention relates to improvedtechniques for ascertaining the end of an etch process for endpointingpurposes while etching through a selected layer on a substrate.

In the manufacture of semiconductor devices, such as integrated circuitsor flat panel displays, the substrate (e.g., the wafer or the glasspanel) may be processed in a plasma processing chamber. Processing mayinclude the deposition of layers of materials on the substrate and theselective etching of the deposited layer(s). To prepare a layer foretching, the substrate surface is typically masked with an appropriatephotoresist or hard mask. During etching, a plasma is formed from theappropriate etchant source gas to etch through regions unprotected bythe mask. The etching is terminated once it is determined that thetarget layer is etched through. This termination of the etch istypically referred to as the etch “endpoint.”

To determine when to terminate an etch, many techniques have beenemployed in the art. By way of example, the etch may be terminated uponthe expiration of a predefined period of time. The predefined period oftime may be empirically determined in advance by etching a few samplesubstrates prior to the production run. However, there is no allowancemade for substrate-to-substrate variations as there is no feedbackcontrol.

More commonly, the end of an etch process may be dynamically ascertainedby monitoring the optical emission of the plasma. When the target layeris etched through, the optical emission of the plasma may change due tothe reduced concentration of the etch byproducts, the increasedconcentration of the etchants, the increased concentration of thebyproducts formed by reaction with the material(s) of the underlayer,and/or due to the change in the impedance of the plasma itself.

It has been found, however, that the optical emission-based techniquehas some disadvantages. By way of example, the use of some etchantsand/or additive gases interferes with the optical emission endpointtechnique, giving rise to inaccurate readings. As a further example, asthe feature sizes decrease, the amount of film exposed to the plasmathrough openings in the mask is also reduced. Accordingly, the amount ofbyproduct gases that is formed from reactions with the exposed filmreduces, rendering signals that rely on plasma optical emission lessreliable.

It has been found that, as the target layer etch is completed and theunderlayer is exposed to the plasma, the self-induced bias of thesubstrate may change. By way of example, for the etch of a dielectrictarget layer, the self-induced bias of the substrate is observed tochange as a conductive underlayer is exposed to the plasma. As a furtherexample, for the etch of a conductive target layer, the self-inducedbias of the substrate is observed to change when a dielectric underlayeris exposed to the plasma. By monitoring the change in the self-inducedbias of the substrate, the end of the etch process may be ascertainedfor endpointing purposes.

To facilitate discussion, FIG. 1 illustrates a typical endpointingarrangement wherein the self-induced bias on the wafer is monitored todetermine when the target layer is etched through for the purpose ofendpointing the etch. As shown in FIG. 1, a wafer 102 is shown disposedon an electrode 104, which is typically made of a metallic material.Electrode 104, which functions as a chuck in this example, is energizedby an RF power source 106 through a capacitor 108. During etching, theself-induced bias on wafer 102 is detected at a node 110 through amonitoring circuit 112. Monitoring circuit 112 include a low pass filter114, which blocks the RF component of the signal and allows only the DCcomponent to pass through. Since the self-induced bias on the wafertends to be in the hundreds of volts, the signal that is passed throughlow pass filter 114 is typically stepped down through a voltage dividercircuit to allow the monitoring electronics (not shown to simplify thediscussion) to monitor the change in the self-induced bias on wafer 102.This information pertaining to changes in the self-induced bias on thewafer allows the endpointing electronics to determine when the etchshould be terminated.

However, the sensitivity and accuracy of the monitoring techniquediscussed in FIG. 1 may degrade as the percentage of the target filmexposed to the plasma decreases and/or if the DC conductivity betweenthe plasma and the electrode is decreased (e.g., due to the presence ofa dielectric layer underlying the target layer to be etched).Furthermore, the monitoring technique of FIG. 1 is typically ineffectivewhen electrostatic chucks are employed. This is because electrostaticchucks typically employ a dielectric layer between the conductive chuckbody and the substrate. The presence of this dielectric layer interfereswith the current path between the plasma and the chuck, rendering itvery difficult to accurately determine the self-induced bias on thewafer at node 110. Furthermore, the relationship between the voltagedetected at node 110 and the self-induced bias on wafer 102 is notlinear. By way of example, the resistance of the electrostatic chuckdepends, in part, on the voltage existing on the chuck. Accordingly evenif a signal can be detected at node 110, it is difficult to correlatethe signal detected with the self-induced bias on the substrate forendpointing purposes.

In view of the foregoing, there are desired improved techniques fordetecting the end of a plasma etch process for endpointing purposes.

SUMMARY OF THE INVENTION

The invention relates to methods and apparatus for ascertaining the endof an etch process while etching through a target layer on a substratein a plasma processing system. This invention exploits the change in theelectric potential of the substrate which, for many different etchapplications, corresponds to the end of the etch process. In oneembodiment, the endpointing arrangement includes a current monitoringcircuit configured to monitor the current flowing to a pole of theelectrostatic chuck to detect a pattern indicative of the end of theetch process. Upon ascertaining the pattern indicative of the end of theetch process in the current signal, a control signal is produced toterminate the etch.

In another embodiment, the chuck represents a bipolar electrostaticchuck and currents flowing to both poles of the electrostatic chucks aremonitored for the aforementioned pattern indicative of the end of theetch process in order to terminate the etch. In yet another embodiment,the differential of the currents supplied to the poles of theelectrostatic chuck is monitored for the aforementioned patternindicative of the end of the etch process in order to terminate theetch.

In yet another embodiment, the electrostatic power supply includes abias compensation power supply, which monitors currents supplied to theelectrostatic chuck poles and outputs a compensation voltage responsivethereto. The compensation voltage is then input into the chuck powersupply in order to keep the currents supplied to the poles substantiallyequal but opposite in sign throughout the etch. In this embodiment, thecompensation voltage is monitored for the aforementioned patternindicative of the end of the etch process in order to terminate theetch.

These and other advantages of the present invention will become apparentupon reading the following detailed descriptions and studying thevarious drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 illustrates a typical endpointing arrangement wherein theself-induced bias on the wafer is monitored to determine when the targetlayer is etched through for the purpose of endpointing the etch.

FIG. 2 is a simplified illustration of a compensation arrangement forkeeping the currents supplied to the chuck poles substantially equal inmagnitude but opposite in sign as the etch progresses.

FIG. 3 illustrates a typical compensation voltage as the etch progressesthrough the target layer.

FIG. 4 illustrates, in accordance with one embodiment of the presentinvention, a simplified arrangement for monitoring the compensationvoltage for the purpose of endpointing the etch.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to avoidunnecessarily obscuring the present invention.

It is appreciated by the inventors herein that as the etch progressesthrough a target layer, and particularly as the target layer is etchedthrough to the underlayer, the electric potential of the substratechanges. The change in the substrate potential is particularlypronounced at the end of the etch. While not wishing to be bound bytheory, it is believed that, as the target layer is etched through, thecapacitive and resistive coupling between the substrate and the plasmachanges. As one possible explanation, the self-induced bias on thesubstrate may change due to the increased current leakage between theplasma and the substrate as the etch features (such as vias or trenches)are etched down to a stop layer. It is also possible that the propertiesof the plasma itself are changed as the target layer is etched through.This change brings about a change in the plasma impedance, which in tunchanges the self-induced bias on the substrate.

When an electrostatic chuck is employed in the plasma processing system,direct measurement of the substrate electric potential is difficult,because the dielectric layer of the ESC introduces a large resistancebetween the substrate and the electrical measurement circuitry. Thepresent invention overcomes these difficulties.

It is appreciated by the inventors herein that changes in the substrateelectric potential cause variations in the current flowing from the ESCpower supply to the poles of the electrostatic chuck. In one of theembodiments of the present invention, the currents flowing to the polesof the electrostatic chuck are monitored. In this manner, the change ofsubstrate potential associated with the end of the etch process may beascertained, and the information derived therefrom may be employed toendpoint the etch.

More preferably, some electrostatic chuck power supplies employ acompensation circuit to keep the currents flowing to the poles of theelectrostatic chucks substantially equal in magnitude but opposite insign. Compensation circuits are employed since if electrostatic forcesbetween the chuck poles and the overlying substrate regions vary duringan etch, inconsistent chucking, inconsistent heat transfer, andundesirable etch results may occur. In some systems, however, thecompensation circuit may be employed to keep the currents flowing to thepoles of the electrostatic chuck substantially constant (i.e.,relatively unchanging even if they are unequal throughout the etch).

In general, the compensation circuit typically monitors the currentsflowing to the poles of the electrostatic chuck and provides a controlsignal to a variable bias compensation power supply. When the currentsflowing to the poles of the electrostatic chuck poles change, thechanging control signal varies the voltage output by a bias compensationpower supply. The voltage output by the bias compensation power supply,referred to herein as the compensation voltage, is then employed tooffset the voltages supplied to the chuck poles in order to keep thecurrents flowing to the electrostatic chuck poles substantially equal inmagnitude but opposite in sign (or substantially constant in othersystems as mentioned earlier).

It is discovered by the inventors that the compensation voltage changesas the etch progresses and typically changes dramatically as the targetlayer is cleared, i.e., etched through. In accordance with oneembodiment of the present invention, information regarding end of theetch process may be obtained by monitoring the compensation voltage inorder to endpoint the etch.

To facilitate discussion, FIG. 2 is a simplified illustration of acompensation arrangement for keeping the currents supplied to the chuckpoles substantially equal but opposite in sign as the etch progresses.It should be kept in mind, however, that while the compensationarrangement of the exemplary embodiment functions to keep the currentssupplied to the chuck poles substantially equal but opposite in sign,the concepts disclosed herein also apply equally to compensationarrangements that keep the currents flowing to the poles substantiallyunchanging (i.e., relatively unchanging even if they are unequalthroughout the etch). The adaptation of the exemplary arrangement towork with such a compensation circuit is well within the skills of oneof ordinary skills in the art given this disclosure.

With reference to FIG. 2, the object to be processed 200, e.g. a waferor glass panel, includes the target layer to be etched, and isrepresented in a simplified manner by a photoresist mask layer 202, atarget layer 204, underlayer film or films 206, and the substrate 207.Target layer 204 may represent any layer to be etched through. In oneexample, target layer 204 represents a silicon dioxide-containing layersuch as a doped CVD (chemical vapor deposition) or PECVD(plasma-enhanced chemical vapor deposition) glass layer. In anotherexample, target layer 204 may represent a low dielectric constant (low-kdielectric) layer. In yet another example, target layer 204 represents ametal layer or polysilicon (doped or undoped) to be etched. Underlayerfilm or films 206 may include any and all layers and/or structures thatunderlie target layer 204. Underlayer film or films 206 may include, forexample, one or more conductive (metallic or doped polysilicon) layersand/or one or more dielectric layers. By way of example, an etch stoplayer may be disposed immediately below target layer 204 and may beformed of, for example, silicon nitride, titanium silicide, or titaniumnitride material. Substrate 207 represents the supporting material ofthe object to be etched, for example, a wafer or glass panel. For thesake of discussion in the present example, substrate 207 does notinclude the layers and/or device structures which may be present on itssurface, which are instead represented by the aforementioned layers 202,204, and 206. In some cases, the underlayer film or films 206 may beabsent, and the target layer 204 is disposed directly on the substrate207.

In the example of FIG. 2 a Johnsen-Rahbek chuck is employed although theinvention is believed to work with any type of electrostatic chuck suchas monopole ESC chucks, multipole ESC chucks of any configuration, orthe like. The construction of a Johnsen-Rahbek chuck is well known inthe art and will not be discussed in detail here for brevity's sake.Further, although the chuck poles are of a concentric configuration inthe example of FIG. 2, the poles of the electrostatic chuck may assumeany configuration and/or geometry (e.g., inter-digitated). For theconcentric Johnsen Rahbek chuck of the example of FIG. 2, an outer pole208 and an inner pole 210 are embedded in a slightly conductive layer212, which may be formed of, for example, a ceramic material that islightly doped for conductivity. An RF electrode 214, which is disposedbelow slightly conductive layer 212, is typically formed of a metallicmaterial and is coupled to an RF power supply 216 through a capacitor218. To facilitate chucking, the poles of chuck 220 are coupled to anelectrostatic power supply 222.

Electrostatic chuck power supply 222 includes a main power supply 224,which supplies the DC chucking voltages to the poles of chuck 220. Lowpass filters to 230 and 232 are interposed between poles 208 and 210 andelectrostatic chuck power supply 222 to couple main power supply 224 topoles 208 and 210 of chuck 220 and to isolate RF power 216 from powersupply 222. Current monitoring circuits 234 and 236 are coupled inseries with the current paths between the poles of the electrostaticchuck and ESC power supply 222 to monitor the currents in these legs.

Each of current monitor circuits 234 and 236 may be implemented by asimple resistive arrangement, and the potential difference across eachmay be ascertained to determine the current flowing to each of poles 208and 210. The outputs of current monitor circuits 234 and 236 are inputinto a comparator circuit 238, which may represent, for example, adifferential amplifier circuit. Comparator circuit 238 outputs a controlsignal 240 for controlling a variable bias compensation power supply242. Bias compensation power supply 242 changes its output responsive tocontrol signal 240. The output of bias compensation power supply 242 isemployed to bias main power supply 224 to keep the currents flowing topoles 208 and 210 substantially equal in magnitude and opposite in sign.The arrangement of FIG. 2, including the bias compensation arrangementin electrostatic chuck power supply 222, is well known in the art.

As target layer 204 is etched through, the compensation voltage at node250 changes as the compensation circuit attempts to keep the currentsflowing to poles 208 and 210 substantially equal. It is appreciated bythe inventors herein that the information contained in the compensationvoltage, which is found either in control signal 240 or at node 250 atthe output of bias compensation power supply 242, includes informationpertaining the progress of the etch and particularly pertaining when theend of the etch occurs. This is because, as explained earlier, theelectric potential of the substrate 207 changes as the etch progresses,and causes the currents flowing to each of the poles 208 and 210 tochange. These changes are detected by current monitor circuits 234 and236 to produce a control signal 240, which serves as the feedback signalto bias compensation power supply 242, whose job it is to bias mainpower supply 224 to keep the currents flowing to poles 208 and 210substantially equal.

FIG. 3 illustrates a typical compensation voltage as the etch progressesthrough the target layer. At point 302, the etch begins on compensationvoltage plot 300. As the etch progress, the compensation voltagechanges. Although the change is illustrated in FIG. 3 by an increasingcompensation voltage, the compensation voltage may change in other ways,such as decreasing, as the etch progresses in other substrates. As theetch clears the target layer, a significant change in the compensationvoltage is typically observed. Although the end of the etch is evidencedby a steep upward slope in the vicinity of region 304 in FIG. 3, the endof the etch may also be evidenced (in other etch processes) by a sharpdownward slope, a spike or a sudden dip in the signal. Irrespective ofthe exact shape of the compensation voltage plot at the time the etchends, the end of the etch is typically evidenced by a clearlydiscernible change in the compensation voltage. The specificcharacteristic shape of the compensation voltage plot at the time theetch ends may be ascertained by performing sample etches on samplewafers. Thereafter, the monitoring circuitry may be instructed to lookfor the ascertained characteristic shape in the compensation plot thatsignals the end of the etch for endpointing purposes.

FIG. 4 illustrates, in accordance with one embodiment of the presentinvention, a simplified arrangement for monitoring the compensationvoltage for the purpose of endpointing the etch. In FIG. 4, the voltageat node 250 is input into endpoint monitoring circuitry 402, whichoutputs an endpoint signal 404 when the characteristic change indicativeof the end of the etch process is ascertained. Monitoring circuitry 402may represent, for example, programmable digital circuitry that has beenprogrammed to analyze the input compensation voltage signal and tooutput a control signal 404 for endpointing the etch process. In oneexample, monitoring circuitry 402 represents a general purpose digitalcomputer (e.g., a microcomputer) or a digital signal processor that hasbeen programmed to analyze the digitized compensation voltage signal forchanges indicative of the end of the etch process.

In accordance with another embodiment of the present invention, it isalso possible to monitor control signal 240 itself for changescharacteristic of the end of the etch for endpointing purposes. Inaccordance with yet another embodiment of the present invention, thecurrents through the legs themselves may be monitored (by, for example,monitoring the outputs of current monitor circuits 234 and 236) forchanges in the current(s) that are indicative of the end of the etchprocess. This latter embodiment is particularly useful for chucks whichdo not employ compensation circuitry.

In accordance with another embodiment of the present invention, thedifference in currents through the pole legs may be monitored indirectlyby the current monitoring circuit 248, even in the absence of powersupply 242.

As can be appreciated from the foregoing, many embodiments of theinvention take advantage of existing signals in the electrostatic chuckpower supply for the purpose of ascertaining when the end of the etchoccurs in order to terminate the etch. In an indirect manner, changes inthe currents supplied to the poles of the electrostatic chuck areemployed to ascertain the etch progress for endpointing purposes. Unlikeprior art techniques, the endpointing technique of the present inventiondoes not require directly monitoring the self-induced bias of thesubstrate through the electrode (as was done in the case of FIG. 1).Accordingly, the technique works even with electrostatic chucks, whichhas a nonconductive dielectric layer disposed between the wafer and thebody of the chuck.

In fact, the accurate determination of when the etch ends is possibleeven if there is a nonconductive layer disposed between the chuck'smetallic body and the target layer. The presence of the nonconductivedielectric layer, either as part of the electrostatic chuck or withinthe substrate, would presumably have caused the prior art endpointingcircuitry of FIG. 1 to fail to accurately provide an endpoint signalsince the prior art technique depends on the direct measurement of theself-induced bias on the substrate through the electrode for endpointingpurposes. Additionally, one of ordinary skills in the art would haveassumed that the presence of a dielectric layer on the surface of theelectrode and/or under the target layer would block the electrical path,rendering the direct monitoring of the self-induced bias on thesubstrate impossible and/or very difficult. Since the present inventiondoes not rely on direct contact between the substrate and the electrode,the presence of a such a dielectric layer does not prevent theascertaining of the end of the etch in the present invention.

It is also observed that the inventive endpointing technique is highlysensitive and is capable of accurately providing endpointing informationeven when etching substrates having a small fraction (or percentage) ofthe target layer exposed to the etching plasma. The sensitivity appearsto increase if a conductive layer, e.g., a conductive metal or dopedpolysilicon interconnect layer, is disposed below the target layer to beetched. As alluded to earlier, the sensitivity of the present techniqueis such that the end of the etch process may be ascertained even ifthere is a dielectric layer disposed under the target layer.Furthermore, since endpointing does not depend on monitoring the opticalemission of the plasma, the inventive technique also works irrespectiveof the etchant and/or additive gas employed.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. In general, it is proposed thatthe endpoint data can be derived from the changes in the substratepotential, which can in turn be obtained by looking at various signalsat various points in the system. Thus, although the endpoint data can beascertained by monitoring the changes in the current(s) flowing to thepole(s) of the ESC chuck (which reflect the changes in the substratepotential), there are other ways of obtaining this substratepotential-based endpoint data when an ESC chuck is involved. By way ofexample, a probe which contacts the backside of the substrate or someappropriate place on the substrate may be employed to measure thesubstrate potential directly throughout the etch, and the probe signalmay be analyzed for changes indicative of the etch termination forendpointing purposes.

As another example, the leakage flow rate of coolant gas from the edgesof the ESC chuck may be monitored during the etch, as an indirectmeasure of the substrate electric potential. This flow rate is dependentupon the clamping force of the ESC, which is, in turn, dependent uponthe potential difference(s) between the ESC and the substrate. As theetch proceeds, detectable changes in the flow rate may arise due tochanges in the substrate potential. In one embodiment, the leakage flowrate may be monitored in conjunction with or as part of a pressurecontrol arrangement which supplies the coolant gas to the interfacebetween the substrate and the ESC. The flow rate signal may be analyzedfor changes indicative of the etch termination, for endpointingpurposes.

In fact, given this disclosure, one of ordinary skills in the art willreadily recognize that changes in the substrate potential impact othersignals at various points in the plasma processing system. With theknowledge imparted by this disclosure, the identification of thepossible signals and locations in a specific plasma processing systemthat may be monitored to ascertain the changes in the substratepotential is well within the skills of one familiar with plasmaprocessing equipment. It should also be noted that there are manyalternative ways of implementing the methods and apparatuses of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

What is claimed is:
 1. A method for ascertaining an end of an etchprocess while etching through a target layer on a substrate in a plasmaprocessing system, said plasma processing system including anelectrostatic chuck having a first pole, a first DC power supply coupledto said first pole for supplying a chucking voltage to said first pole,a first current monitoring circuit coupled between said first pole andsaid first DC power supply for monitoring a first current supplied tosaid first pole, said first current monitoring circuit outputting afirst signal indicative of said first current, and a variable DC powersupply configured to output a compensation voltage for biasing saidfirst DC power supply responsive to said first signal, thereby causingsaid chucking voltage to vary responsive to said compensation voltage,said method comprising: coupling an endpoint monitoring circuit to saidvariable DC power supply, said endpoint monitoring circuit having anendpoint monitoring input and an endpoint monitoring output; receivingat said endpoint monitoring input said compensation voltage; analyzing,using said endpoint monitoring circuit, said compensation voltage for apattern characteristic of said end of said etch process; and outputtingat said endpoint monitoring output an endpoint signal indicative of saidend of said etch process upon ascertaining said pattern in saidcompensation voltage.
 2. The method of claim 1 wherein saidelectrostatic chuck includes a second pole coupled to said first DCpower supply, said plasma processing system includes a second currentmonitoring circuit coupled between said second pole and said first DCpower supply for monitoring a second current supplied to said secondpole, wherein said compensation voltage output by said variable DC powersupply is responsive to both said first signal and a second signaloutput by said second current monitoring circuit, said second signalbeing indicative of said second current.
 3. The method of claim 2wherein said plasma processing system includes a differential amplifierarrangement coupled to said first current monitoring circuit and saidsecond current monitoring circuit, said differential amplifierarrangement receives said first signal and said second signal as inputand outputs a control signal to said variable DC supply to cause saidcompensation voltage output by said variable DC power supply to varyresponsive to both said first signal and said second signal.
 4. Themethod of claim 2 wherein said first signal and said second signal isemployed by said variable DC power supply to maintain said first currentand said second current substantially constant during said etch process.5. The method of claim 2 wherein said endpoint monitoring circuitincludes a general purpose microcomputer.
 6. The method of claim 2wherein said electrostatic chuck represents a Johnsen-Rahbek chuck. 7.The method of claim 2 wherein said substrate includes a conductive layerunderlying said target layer.
 8. The method of claim 2 wherein saidsubstrate includes a dielectric layer underlying said target layer. 9.The method of claim 2 wherein said target layer represents a silicondioxide-containing layer, said substrate further includes a dielectriclayer underlying said target layer.
 10. The method of claim 2 whereinsaid target layer represents a low dielectric constant layer, saidsubstrate further includes a dielectric layer underlying said targetlayer.